Traveling wave maser



Sept. 25, 1962 H. sExDEL TRAVELING WAVE MASER 2 Sheets-Sheet 1 Filed Sept. 9, 1959 we p NU E

E A Ws T Sept. 25, 1962 H. sEIDEL TRAVELING WAVE MASER 2 Sheets-Sheet 2 Filed Sept. 9, 1959 .T j AT ORNEV Patented Sept. 25, 1952 t" free 3,056,091 TRAVELING WAVE MASER Harold Seidel, Fanwood, NJ., assigner to Beil Telephone Laboratories, Incorporated, New York, NX., a corporation of New York Filed Sept. 9, 1959, Ser. No. 838,883 Claims. (ci. sse-4) This invention relates to masers and more particularly, to nonreciprocal traveling wave masers.

The invention is based on my discovery that, with a proper choice of waveguide dimensions, modes of the ferrite dielectric type can be used to propagate energy in the form of a slow wave in a manner which is advantageous in some areas of technology. In particular, it is found that under certain conditions the `group velocity of the propagating modes can be reduced by making the guide smaller, the reduction being proportional to the scaling factor. A slow wave structure of this sort has a natural application, for instance, in the design of stable, high-gain masers where the inclusion of slow wave structures is `dictated Iby the need to extend the interaction time of the input signal and the maser medium so that a high degree of amplification can be obtained in a reasonable leng-th of Wave guide. In addition, the nonreciprocal propagation characteristics of the ferrite dielectric mode can be utilized advantageously to provide lthe degree of stability required in a high-gain amplifier.

Workers in the waveguide art have known for some time that when an appropriately magnetized 'ferrite slab is loaded into a rectangular Wave guide `a series of new modes appear which propagate at large values of the phase constant These modes are not associated with the normal mode, and over a large frequency range propagation in the wave guide is exclusively in the extraordinary modes which have been recognized to be nonreciprocal. Propagation in the particular mode employed in this invention, in fact, occurs in only one direction until the ferrite slab almost fills the interior of `the guide. A-t a critical point propagation in the trst direction is cut off and propagation in the opposite direction is established. The term ferrite ydielectric mode has been used to describe these one-way surface modes in which the magnetic field appears to be concentrated around the yferrite-metal and ferrite-air interfaces. This term is derived from the analogous propagation of surface modes along dielectric materials.

The theory of Iferrites in rectangular wave guides, with some reference to the ferrite dielectric modes, is the subject of an article by Button and Lax in I.R.E. Transactions, Volume AP4, pages 531-537 (1956). As the theoretical background which will be helpful in understanding the present invention is adequately set -forth in the literature, a :detailed account is not included in this specification.

Ordinarily, slow wave propagation is obtained through the use of periodic structures. Included in this description are the helix types which are thought of in terms of geometric slowing, and the cavity types which are thought of in terms of dispersion. There are others, in a sense hybrid to these two types, but nevertheless within the classification of periodic structures. Such periodic structures have generally required precision fabrication to close tolerances. Thus they have been costly to manufacture in addition to being subject to deviations from the calculated and desired characteristics. Furthermore, when periodic structures are used in masers it is generally found necessary to introduce complications to `furnish a degree of nonreciproci-ty or other means of providing for amplifier stability.

An object of the present invention is, therefore, a new and improved nonreciprocal traveling wave maser.

Another object of this invention is a compact and uncomplicated nonreciprocal maser amplier that can be easily constructed with minimal requirements of precision in the process of manufacture.

A feature of the present invention is an arrangement which includes a -section of conductively bounded hollow wave 4guide with a ferrite slab in contact with part of the inner surface thereof.

A typical embodiment of the invention is in a simplified maser which comprises a section of conductively bounded hollow wave guide of appropriate dimensions with a slab of gyromagnetic material partially filling the enclosed volume and With the active maser material disposed in that part of the contained space which is not occupied by the ferrite.

I have discovered that gyromagnetic materials can be loaded into wave guides so that energy will propagate in the lferrite dielectric mode in such a manner that the phase constant =(w) exhibits singulari-ties which may be designated as -oo and -l-oo. Since the group velocity of the wave energy propagating in the guide may be expressed as Vg-d it [follows that for certain values `of w intermediate the singular points the wave is slowed. Furthermore, I have discovered that singularities of opposite sign may be produced by arranging the -gyromagnetic material so that there are at least two magnetically different kinds of interfaces extending along the longitudinal dimension of the wave guide.

In a preferred embodiment in accordance with the invention, a rectangular slab of gyromagnetic material, advantageously yttrium iron garnet, is positioned inside a hollow rectangular Wave guide in contact with one of the walls thereof in such a manner as to fill about onehalf of the volume enclosed by the guide. The width of the guide is made small in comparison with the free space wavelength of the signal to be slowed, and means are provided at each of its ends for coupling the small guide to the larger size generally used to transmit ordinary modes so that the signal energy can be applied to the input end of the device and abstracted at the output end.

The invention will be more fully understood from the .following more `detailed description, taken in conjunction with the accompanying drawing, in which:

FIG. l `shows the basic elements of a slow wave structure in accordance with the invention;

FIG. 1A is a plane section through the device 10 along the line 1A;

FIG. 2 is a plot of the relative intensity of the magnetic field inside the ferrite loaded small rectangular wave guide shown in FIG. 1;

FIG. 3 illustrates diagrammatically one form of a maser incorporating a slow wave structure according to the invention; and

FIG. 3A is a plane section through the device 30 along the line 3A.

Referring now more specifically to the drawing, there is shown a nonreciprocal slow wave structure l0 cornprising a conductively bounded hollow rectangular wave guide 12 with a slab or block of gyromagnetic material 14 in part of the enclosed space and in contact with one of the narrow walls thereof. Following the principles of the invention it has been found that optimum slowing conditions occur in this embodiment when the gyromagnetic member substantially fills the interior of the waveguide on one side of a plane dividing theenclosed space into two approximately equal rectangular volumes. In accordance with the invention, the width of the guide is made much smaller than the free space wavelength of the signal. This choice of dimensions is intended to cut off all of the conventional TE modes. The TE ferrite dielectric modes, however, may still propagate, their ability to do so being independent of the dimensions of the guide, although the characteristics of propagation do depend on the guide size. In particular, I have discovered that if the wave guide is made arbitrarily small the group velocity of the ferrite dielectric mode is reduced in proportion to the scaling factor. The group velocity is found to have a maximum at some defined center of the transmission band and to decrease uniformly to zero at both edges of the band.

Although it is obvious that the group velocity is continuously variable over the operational range of the slow wave structure, most embodiments of the invention will normally seek the highest possible slowing factor. Thus the width of the guide will generally be made as small as is convenient in the particular circumstances. The reduction in size is limited only by the increased losses in the ferrite and the difiiculty of transferring energy to a very small wave guide. To obtain a usable reduction of the group velocity, the waveguide width typically will be no more than about one-half, and preferably no more than about one-eighth, of the free space wavelength. Practical considerations usually make it desirable that the waveguide width be no smaller than about onetwentieth of the free space wavelength. Transfer of signal energy between the small guide and the guide used to propagate the conventional modes is effected by the coupling arrangements 18 and 19. In the illustration the waveguide wall in contact with the ferrite member is extended and broadened while the other three walls are tapered from the small guide up to the standard size. The gyromagnetic member may be extended into the larger guide to achieve a more efficient transfer. Advantageously, it may be tapered to avoid undesirable discontinuities. Thus the member 14 has constant thickness while its width increases with the guide width in sections 18 and 19. It is extended into the standard guide and tapered off in a convenient length. These principles are known in the waveguide art but under ordinary circumstances are not called upon to connect guides which differ so greatly in size.

Since the degree of slowing is also proportional to the magnetization of the slab 14, the gyromagnetic material should usually be selected to have a high magnetic moment at the temperature range in which the device is to be operated. A well-known ferrimagnetic substance which has suitable characteristics over a wide thermal range, including the liquid helium temperatures often associated with masers, is yttrium iron garnet. With a polycrystalline form of this material as the slowing medium and with a steady magnetic eld of 600 gauss the internal magnetization is about 1700 gauss and the accompanying useful bandwidth is of the order of 2 kmc. In a wave guide having a width of 1A inch and a height of lx inch, such an arrangement yielded a center-band slowing factor of about 10.

An illustrative embodiment of the invention is depicted in FIG. 3, wherein there is shown a maser 3) comprising a conductively bounded hollow rectangular wave guide 32 with one of the narrow walls replaced by a ridge 36 which extends into the space enclosed by the guide and has its sides close to the broad faces thereof. The gyromagnetic material 34 is positioned along the other narrow wall and partially fills the contained space. The maser medium 38 is disclosed in the space between the ridge 36 and the gyromagnetic material 34, the most advantageous location being determined from data of the kind illustrated in FIG. 2 where is the coordinate of the ferriteair interface in the structure shown in FIG. 1. As can be seen from the curve, most of the flux of the radiofrequency magnetic eld is close to the face of the gyromagnetic member.

CII

In the illustrated embodiment 36 the medium 38 typically comprises a paramagnetic solid characterized by at least three discrete energy levels. Desired separation of the energy levels may be attained by means of the Zeeman effect due to a D.C. magnetic field Hdc extending through the medium 38. In order to maintain the clarity of the drawing the means for establishing the magnetic field are not shown but its direction is indicated by the arrow labeled Hdc. The same field may also be utilized to establish the sense of magnetic properties of the gyromagnetic member, though in some cases it may be desirable to use different values of Hdc for each of these purposes, in which case magnetic shunting can be utilized to achieve the desired distribution of magnetic field.

To operate the maser, pump energy is supplied to the medium 38 at a frequency corresponding to the separation between the appropriate Vpair of the energy levels. By these means a population inversion with respect to a second pair of energy levels is produced in the crystal. Such an inversion is known in the maser art as a negative temperature. The signal input is then made to correspond to the separation between such second pair of energy levels. Interaction between the signal and the medium stimulates the return transition and energy of the corresponding frequency is radiated, resulting in amplication of the input. A high degree of amplification is achieved by means of the extended interaction time of the slow wave with the maser medium. Additionally, the ferrite dielectric mode lends stability to the amplifier by virtue of its nonreciprocal characteristics.

Generally, the width W of the wave guide 32 will be too small to support the pumping wave in the absence of any modification in the basic rectangular geometry. In order to carry the pump energy to the medium 38, one of the narrow walls of the guide is replaced by the ridge 36. The distance S--W is substantially V2A at the pumping frequency, although in most cases W will be somewhat smaller than S and may be disregarded.

If the maser is tuned by varying the applied D.C. magnetic field, the slowing medium will be tuned simultaneously. While the bandwidth of the gyromagnetic member decreases proportionally to the slab magnetization, in most cases likely to occur it considerably exceeds the bandwidth of the main resonance line of the maser crystal.

Various modifications are, of course, feasible within the spirit of the invention, both in the slow wave structure of FIG. 1 and in the maser shown in FIG. 3.

In particular, the shape of the wave guide is not restricted to the rectangle shown in the illustrative embodiments. Other possible configurations will be apparent to those skilled in the art. Additionally, the maser material and the gyromagnetic medium may each take a variety of shapes and forms, the better to adapt the device for operation at a particular frequency or power level. Following the principles of the invention, a plurality of members having different magnetic properties may be placed in the wave guide with the gyromagnetic member or members and contiguous thereto so as to create the necessary longitudinal singularities in In such an arrangement the ferrite-metal interface is not required and the slab need not be in contact with the waveguide wall. Furthermore, a variety of coupling or matching techniques may be used to transfer `energy to and from the small wave guide.

What is claimed is:

l. A nonreciprocal maser amplifier comprising a conductively bounded hollow waveguide, an elongated gyromagnetic member disposed within said waveguide, said member extending longitudinally of said waveguide and partially lling the interior thereof, said member having a first longitudinally extending surface portion in contact with and conforming to an interior surface portion of said waveguide, means forming a negative temperature medium disposed vwithin said waveguide in the space not occupied by said gyromagnetic member, a substantial Volume of said negative temperature means extending into the region immediately adjacent a second surface portion of said gyromagnetic member, said negative temperature means being characterized by an energy level system adapted to amplify signals having a frequency below the cutoif frequency of said waveguide, means for establishing a unidirectional transverse magnetic eld through said gyromagnetic member, means for applying pump energy to said negative temperature means, means connected to one end of said waveguide for causing signal wave energy to propagate therethrough in a slow ferrite dielectric mode, and means for abstracting an amplified signal output from the other end of said waveguide.

2. A device as in claim 1 wherein said gyromagnetic member comprises yttrium-iron garnet.

3. A nonreciprocal maser amplier as in claim 1 in which said waveguide is of rectangular cross-section, and said gyromagnetic member has a thickness substantially equal to the height of said waveguide and a width substantially smaller than the width of said waveguide.

4. A nonreciprocal maser amplifier as in claim 1 wherein said negative temperature means comprisesy a paramagnetic solid and said gyromagnetic member comprises a ferrimagnetic solid.

5. A nonreciprocal maser amplifier comprising a conductively bounded hollow rectangular ridged waveguide, an elongated gyromagnetic member disposed within said waveguide, said member extending longitudinally of said waveguide and partially iilling the interior thereof, said member having a first longitudinally extending ilat surface portion in contact with the waveguide wall opposite the interior face of the ridge and a second longitudinally extending at surface portion spaced from the interior face of the ridge, means forming a negative temperature medium disposed within said waveguide in the space between said gyromagnetic member and the interior face of the ridge, said negative temperature means being characterized by an energy level system adapted to amplify signals having a frequency below the cutoff frequency of a waveguide having a width equal to the distance between the face of the ridge and the opposite side of said waveguide, means for establishing a unidirectional transverse magnetic field through said gyromagnetic member, means for causing a pump wave to propagate through said ridge `waveguide for inverting the population distribution in said negative temperature means, means connected to one end of said waveguide for causing signal Wave energy to propagate therethrough in a slow ferrite dielectric mode, and means for abstracting an amplified signal output from the other end of said waveguide.

References Cited in the tile of this patent UNITED STATES PATENTS Tien Apr. 2l, 1959 OTHER REFERENCES 

